Keeping pace with the synthetic advancement of colloidal semiconductor quantum dots (QDs) towards high-quality fluorescence, QD-light-emitting diodes (LEDs) on the fundaments of color conversion and electroluminescence (EL) have substantially progressed for last two decades. In the case of color-conversion QD-LEDs, blue InGaN LED chip is the most common source for exciting various QDs in the emission range of green-to-red, whose choice is dependent on the targeted application. For the demonstration of display device with a high color reproducibility, only two colored QDs of highly color-pure green and red emissions are chosen. Meanwhile, in the lighting device that requires a spectral coverage as large as possible to achieve a high color rendering property, QDs with a broad emission character and/or multiple QDs with various color combinations is used, Jang et al. reported that a combination of green- and red-emitting CdSe/multiple-shell QDs and a blue InGaN LED exhibits the luminous efficacy of 41 lm/W in the operating current of 20 mA [2010 Adv. Mater. 22 3076]. According to Wang et al., a combination of green-, yellow- and red-emitting CdSe/multiple-shell QDs and a blue LED exhibits luminous efficacy of 32 lm/W and color rendering index of 88 in the operating current of 40 mA [2011 J. Mater. Chem. 21 8558]. However, the harmful Cd substance in QDs is not desirable when manufacturing sustainable next-generation products without environmental degradation.
Among non-Cd composition candidates capable of visible emission, there are III-V compound InP and II-VI compound ZnSe. However, these compounds with composition have problem with intricate production for accurately achieving desired wavelengths.
I-III-VI type chalcogenide is another promising class of non-Cd visible QD emitters. Through the variation of band gap of QDs by means of control of composition, size, and cationic off-stoichiometry, a wide emission range of green-to-deep red has been attainable.
Yellow-emitting Cu—In—S (hereinafter CIS) or Ag—In—S (hereinafter AIS) QDs have been successfully combined as single downconverters with a blue InGaN LED to produce a white electroluminescence (EL), but the resulting white LEDs possessed only moderate CRIs around 70. This limited color rendering property is primarily attributable to the cyan and red deficiencies in the bicolored (i.e., blue plus yellow) EL distribution. In our earlier investigation, two emitters of greenish-yellow (546 nm) CIGS and orange (578 nm) CIS QDs were integrated with a blue LED. The resultant CRIs were higher than those of single QD-based white devices above, but were not satisfactorily high, not exceeding 80, since PLs of such two QD emitters were spectrally overlapped to a substantial extent and thus not separated sufficiently to secure a large spectral coverage [W. S. Song, J. H. Kim, J. H. Lee, H. S. Lee, Y. R. Do and H. Yang, J. Mater. Chem., 2012, 22, 21901-21908].
It will be desirable if a combination of green and red QDs, most commonly used in color combinations, in particular, non-Cd type QDs, with a blue LED can yield high-color rendering white lighting.
Chen et al. synthesized two different-colored downconverters of green Zn—Cu—In—S (hereinafter ZCIS) (525-535 nm) and red (615 nm) CIS QDs and fabricated low- (20 mA) and high-power (350 mA driving) white QD-LEDs that yielded 95 and 90.5 in CRI and 70 and 10-15 lm/W in luminous efficacy, respectively. Later, based on a wisely chosen combination of green (501 nm) Zn—Ag—In—S (hereinafter ZAIS) and red (606 nm) ZCIS QD emitters ultrahigh color-quality white QD-LEDs with the record CRIs of 94-97 and the reasonable luminous efficacies of 26.7-39.6 lm/W were devised. [H. C. Yoon, J. H. Oh, M. Ko, H. Yoo and Y. R. Do, ACS Appl. Mater. Interfaces, 2015, 7, 7342-73 50].
A strong vantage point of the above color-conversion QL-LEDs is the unnecessariness of blue QD, rendering their commercialization more viable. On the other hand, EL-QD-LED should have a blue QD emitter as a primary color for the realization of full-color emission for display and white emission for lighting device.
Blue QDs have been synthesized mostly with Cd-containing II-VI compositions such as CdZnS, CdZnSe, and CdZnSeS and then successfully utilized as active emitting components for the fabrication of mono- and multi-chromatic EL-QD-LEDs. However, the harmful Cd substance in such blue QD elicits a grave concern in applying them for the manufacturing of sustainable next-generation products without environmental degradation as mentioned previously.
Among a few non-Cd composition candidates suitable for synthesis of visible QD emitters, III-V InP is the most studied one, particularly since optimally synthesized InP QDs are comparable to well-developed Cd QDs with respect to photoluminescence (PL) quantum yield (QY) and emission bandwidth. However, synthesis of InP QDs has been highly weighted towards green-to-red colors with blue emitters nearly unexplored. Given a relatively low-energy bulk band gap (1.35 eV at room temperature) of InP, InP QDs should be ultrasmall-sized or placed in a highly strong quantum confinement regime to produce a high-energy blue emission, which in turn makes their controlled synthesis highly challenging. Nevertheless, a few blue InP QDs with emission peak wavelengths of <480 nm were developed, but the resulting QDs possessed unsatisfactory QYs of 5-40%, depending on their own core/shell structural details.
Another QD composition candidate for non-Cd blue emitters is II-VI ZnSe, whose band gap (2.69 eV) is significantly higher than that of InP. This implies that ZnSe QDs should be relatively large-sized to obtain relevant blue color wavelengths, which may be also a tough task in the practical QD synthesis. For that reason, most of ZnSe QDs to date have exhibited intermediate wavelengths between violet and blue color.
I-III-VI type chalcogenide is another promising class of non-Cd visible QD emitters. Through the variation of band gap of QDs by means of control of composition, size, and cationic off-stoichiometry, a wide emission range of green-to-deep red has been attainable. However, highly fluorescent I-III-VI QDs capable of emitting shorter wavelengths than ca. 500 nm (corresponding to greenish cyan) have not been demonstrated yet.